CN107943130B

CN107943130B - Control device with current protection solid state relay

Info

Publication number: CN107943130B
Application number: CN201710947072.0A
Authority: CN
Inventors: C·S·尼库莱斯库; J·T·麦斯特尔
Original assignee: Ecobee Inc
Current assignee: Ecobee Inc
Priority date: 2016-10-13
Filing date: 2017-10-12
Publication date: 2021-05-25
Anticipated expiration: 2037-10-12
Also published as: US20190338970A1; CN107943130A; US20180106489A1; US20180149381A1; US10488063B2; CA2981023C; US10215432B2; US11187423B2; CA2981023A1

Abstract

A system for a control device to enable and disable its control using a solid state relay as a switch. The method includes monitoring current flowing through at least some of the solid state relays, determining an overall amount of heat generated in the solid state relays and their associated circuitry and printed circuit board wiring, and adding the overall amount of heat to other determined amounts of overall heat and using it to compensate for readings provided by temperature sensors within the control device affected by the overall heat. By measuring the current flowing through the power bus to one or more of the solid state relays of the control device, a damaging overcurrent condition will be distinguished from an allowable transient overcurrent condition, and the control device can disable any solid state relays that may be damaged while allowing the solid state relays that are experiencing the allowable transient to remain operational. In the event of a severe overcurrent condition, the current monitoring device issues a fault signal, triggering an interrupt condition, causing the processor in the controller to close the affected solid state relay very quickly.

Description

Control device with current protection solid state relay

Technical Field

The present invention relates to a control device. More particularly, the present invention relates to a control device, such as an environmental control device for an HVAC system or the like, wherein the control device employs solid state relays to activate and deactivate subsystems controlled by the device.

Background

There are control devices for many systems. A common control device with which most people are familiar is an environmental control device, such as an HVAC thermostat, that can control a variety of environmental factors (heating, cooling, humidity, ventilation, etc.) at their home or other location. While thermostats have been in use for many years, until recently such controls have been simple analog/mechanical devices employing sensors such as bimetallic strips in combination with mercury tilt switches that directly activate or deactivate relays, contactors or other HVAC controls.

Smart thermostats have now been developed that employ a digital processor executing potentially complex software programs to better controlAnd (4) environmental factors are produced. These intelligent thermostats are typically equipped with a variety of sensors (solid state temperature and humidity sensors, etc.) and other information (from remote sensors that provide occupancy information and/or remote temperatures, etc. and/or from network-connected servers that provide weather conditions and forecasts, etc.) that provide inputs to the executing software to control the respective HVAC system. Such intelligent thermostats are becoming increasingly popular because they typically allow remote control and monitoring of the operation of the intelligent thermostats and conditions in the controlled environment (typically via internet applications), and because they provide increased user comfort and/or reduced HVAC system energy usage. Examples of such intelligent thermostats include Ecobee (250University Avenue, Suite 400Toronto, ON, Canada, M5H 3E5) manufactured by Ecobee₃And a controller.

While such intelligent thermostats represent a significant improvement over existing thermostats, their design, manufacture and operation present unique challenges. For example, HVAC control systems typically require switching of large amounts of electrical power to activate air conditioning compressors, circulation fans, and the like. While prior art analog/mechanical thermostats can typically handle significant levels of electrical power, including various abnormal conditions (faults), in contrast, digital controls are more susceptible to spikes, over-voltage, etc., and are still expected to provide reliable service over the years without faults. Furthermore, while the advantages of intelligent thermostats are obvious, consumer behavior still requires that intelligent thermostats be affordable, relatively small in size, and most importantly reliable. Meeting all these criteria is a difficult task.

Disclosure of Invention

It is an object of the present invention to provide a new control device which obviates or mitigates at least one of the disadvantages of the prior art.

According to a first aspect of the present invention there is provided a control apparatus for controlling one or more electrical subsystems according to a control program, comprising: a housing; a memory storing an operating program; at least one environmental sensor within the housing for determining a temperature within the housing; at least one solid state relay operable when enabled to connect at least one electrical subsystem to a power source and further operable when disabled to disconnect the at least one electrical subsystem from the power source; at least one current measurement device for determining an amount of current flowing from the power source to the at least one electrical subsystem when the at least one solid state relay is enabled; and a processor operable to execute the operating program to: determining an overall amount of heating generated by the determined amount of current and the associated predetermined resistance; adding the determined total heating amount to the total heating amount generated in the housing by other components to obtain a determined sum of the total heating in the housing; compensating the determined temperature value by an amount corresponding to the determined sum of the bulk heating in the housing to obtain a compensated determined temperature value; and activating and deactivating the at least one solid state relay to activate and deactivate the at least one electrical subsystem as needed to maintain the compensated determined temperature value within a preselected desired temperature value range.

According to another aspect of the present invention, there is provided a method of controlling an HVAC system comprising at least two subsystems using a control device: measuring a temperature using a temperature sensor located within a housing of the control device; determining a sum of the overall heating produced by the components and power flow within the housing, comprising the steps of: (a) measuring current flowing through at least one solid state relay to at least one of the at least two HVAC subsystems; (b) determining an overall heating value corresponding to the measured current; (c) determining an overall heating value corresponding to overall heating by other components within the housing; (d) summing all of the determined bulk heating values to obtain a total bulk heating value; (e) compensating the measured temperature in view of the determined total overall heating value (to correct for) to obtain a compensated temperature value; comparing the compensated temperature value to a preselected temperature value range; and activating and deactivating the at least one solid state relay to activate and deactivate the at least one HVAC subsystem as needed to maintain the compensated temperature value within the preselected temperature value range.

According to another aspect of the present invention there is provided a method of protecting at least one solid state relay connected between a load and a power bus within a control device from potentially damaging overcurrent conditions, comprising the steps of: (a) determining a set of at least three safety thresholds, each threshold comprising a maximum current level and its maximum allowed time, the thresholds defined in terms of a safe power consumption capability of the solid state relay; (b) measuring a current flowing through the solid state relay; (c) comparing the measured current to each of the at least three safety thresholds; (d) determining whether the measured current exceeds a maximum current level of at least one of the at least three safety thresholds; (e) if the measured current exceeds the maximum current level of the at least one of the at least three safety thresholds, determining if the current has existed for a period of time that exceeds a maximum allowed time defined for that threshold, and if the maximum allowed time threshold has been exceeded, closing each of the at least one solid state relays; (f) repeating step (e) for each of the at least three safety thresholds; and (g) repeating steps (b) through (f).

Drawings

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a schematic diagram of a control for an HVAC system;

FIG. 2 is a block diagram of components of the control device of FIG. 1;

FIG. 3 is a block diagram of a relay assembly of the control device of FIG. 1;

FIG. 4 is a schematic illustration of the current flowing through the output of the control device of FIG. 1; and

fig. 5 is a schematic diagram of the power supply, the turn-off circuit, and the FET of the solid-state relay used in the control device 20.

Detailed Description

A control device according to the present invention is generally indicated at 20 in fig. 1. The control device 20 includes a housing 24 having a front face 28, the front face 28 including at least a transparent portion 32, and a touch screen 36 viewable through the transparent portion 32 and interactive with the touch screen 36. The front side 28 may also include a motion sensor 40, the motion sensor 40 being capable of functioning as an occupancy sensor that detects the presence of a user and/or proximity to the control device 20. A Printed Circuit Board (PCB) is located within the housing 24 and most or all of the hardware of the control device 20 is mounted on or to the PCB, as will be discussed in more detail below.

The touch screen 36 may display a variety of information including operational messages, icons, controls, menus, and the like. For example, the touch screen 36 may display: an icon 44 indicating the current mode of operation (such as heating or cooling); measurements such as humidity level 48 and temperature 52; a user interface element, such as an input slider 56, to allow entry of a desired parameter, such as temperature, or a desired range of parameters; a message window 60 in which relevant messages can be displayed to the user; and other user

interface control icons

64, 68, and 72 that may invoke additional menus (64), information (68) such as outdoor weather, and additional setup menus (72).

It will be apparent to those skilled in the art that the touch screen 36 may be configured and used in a variety of ways in addition to the examples described above, to provide relevant information to a user and/or to allow a user to interact with the control device 20 in a variety of ways.

Fig. 2 shows a block diagram of the hardware of the control device 20, most or all of which may be mounted on a PCB 42. The control device 20 includes at least one processor 100, which may be a microcontroller, microprocessor or any other suitable device as will occur to those of skill in the art, a non-volatile RAM 104 and a volatile RAM 108. It will be apparent to those skilled in the art that either or both of RAM 104 and RAM 108 may be integrated with processor 100 or may be separate discrete devices or components, as desired.

Generally, the non-volatile RAM 104 will store one or more programs for execution by the processor 100 and various parameters associated with execution of the programs, while the volatile RAM 108 will store data and operational values required by the programs.

The touch screen 36 is operatively connected to the processor 100 as is the motion sensor 40, and the control device 20 also includes a real-time clock, not shown, which is an auxiliary device provided in the processor 100, or as a separate component.

The control device 20 further comprises at least one environmental sensor 112, which environmental sensor 112 is at least a temperature sensor, but may also comprise other environmental sensors, such as a humidity sensor, which determine the respective environmental conditions to be controlled and/or monitored. Typically, the environmental sensors 112 in the control device 20 will include at least a temperature sensor and a humidity sensor.

The wireless communication module 116 is operatively connected to the antenna 120 and the processor 100 to allow the processor 100 to communicate with a communication network, such as the internet, and/or additional external sensors (not shown) via at least one wireless communication protocol, such as WiFi, bluetooth, ZigBee, Zwave, cellular data, and the like.

It is specifically contemplated that the wireless communication module 116 will allow at least one remote sensor, and preferably more than one remote sensor, to determine and report the temperature and/or humidity at other locations within the controlled premises in which the control device 20 is installed, and preferably the temperature of the environment external to those controlled premises.

The wireless communication module 116 also allows the control device 20 to communicate with internet-based services, such as weather servers, remote monitoring systems, data logging servers, etc., as well as applications used remotely by a user of the control device 20 to monitor and control environmental conditions at the controlled location.

It will now be apparent to those skilled in the art that the control device 20 operates to perform its programming and to monitor various environmental factors and other conditions and data and to compare these factors and conditions to a set of desired values or range of desired values that are typically specified by a user of the control device 20. When the monitored value changes from the desired value by more than a selected amount, the control 20 will operate one or more appropriate subsystems of the HVAC equipment of the site to change one or more corresponding environmental factors to be closer to the desired value.

For example, if the temperature in the controlled site is below a user-selected target temperature, the control device 20 may activate a furnace (or other heating system) to raise the temperature, and then deactivate the HVAC equipment when the target temperature is reached. If the temperature in the controlled site is higher than the target temperature, the control device 20 may activate an air conditioning system (or a ventilation system or the like) to reduce the temperature or the like.

Accordingly, the control device 20 also includes a relay assembly 140 to provide suitable control signals to the HVAC subsystem or any other device controlled by the control device 20, and the relay assembly 140 is operatively connected to the processor 100 via the signal bus 122 such that the processor 100 can change the output state of the relay assembly 140 (described in more detail below). The relay assembly 140 is employed in the control device 20 because at least some of the outputs of the relay assembly 140 require a greater level of power and/or a different voltage than the output that the processor 100 can directly provide.

Although the relay assembly 140 is illustrated herein as a single subsystem for purposes of illustration and clarity, it should be understood that the relay assembly 140 is not so limited and, in fact, the functionality of the relay assembly 140 may be implemented by suitable components mounted at various locations on the PCB 42 and connected as desired by suitable PCB wiring.

If the control device 20 is used to control the operation of an HVAC system, such a system typically includes a plurality of control signal lines and power lines, often six or more. For the power cord, some HVAC systems have a single transformer that outputs 24VAC for both heating and cooling operations, while other HVAC systems have separate heating and cooling transformers, each of which outputs 24 VAC. In the latter case, as shown in fig. 3, power line "Rh" is the "hot" side of the heating related transformer of the HVAC system connected to control device 20, and power line "Rc" is the "hot" side of the cooling related transformer connected to control device 20.

In the former case of an HVAC system having a single transformer, in the present invention, the hot side of the transformer may be internally bridged by an optional jumper 142 to provide both the Rc and Rh power lines within the control device 20. In many cases, the Rc power line or the Rh power line may also be used to supply power to the control device 20, if desired, or a separate power source may be employed. In fig. 3, the Rc line is used to power the rest of the control device 20.

Common lines (not shown) are connected between one or more other sides of a transformer (or transformers if two or more transformers are present) and the control device 20 to complete the power circuit.

Typical control signal lines from the control device 20 may include: a "G" line for controlling the air circulation fan; an "O/B" line for controlling a heat pump reversing valve; a "Y" line (or Y1 line and Y2 line) for controlling the A/C or heat pump compressor; a "W" line (or W1 line and W2 line) for controlling a heating furnace or an auxiliary heater of the heat pump; and "ACC" lines for controlling accessories such as dehumidifiers, humidifiers, ventilators, etc. In fig. 2, five output terminals (G, O/B, Y, W, ACC) are shown on relay assembly 140, but different terminals and/or a greater or lesser number of output terminals may also be provided as desired.

While the relay assembly 140 may employ electromechanical relays to energize or de-energize the respective terminals, this is not preferred for a number of reasons, including: operational noise; power consumption and heat generation; reliability; size; and expense. In contrast, in the illustrated embodiment, relay assembly 140 employs a solid state relay, such as a FET-based relay, to provide control signals to terminals or remove control signals in response to control signals from processor 100.

One of the known factors that may affect the operation and accuracy of the control device 20 is the self-heating generated by the operation of the control device 20. For example, the processor 100, the wireless communication module 116, the backlighting of the touch screen 36 (which is preferably equipped with a backlighting system), and the relay assembly 140 all generate waste heat (commonly referred to as bulk heating) as an inevitable byproduct of their operation. This overall heating occurs within the housing 24 of the controller 20, and thus the environmental sensor 112 (which is also located within or on the housing 24) will typically report a higher temperature than the ambient temperature in the environment outside the housing 24, resulting in poor or erroneous temperature control of the control device 20.

It is therefore known to apply a compensation factor to the temperatures measured by the internal sensors of the control device to remove the effect of the overall heating on these temperature measurements. One such compensation system is described in U.S. patent 9,016,593 to Metselaar, assigned to the assignee of the present invention.

In the Metselaar patent, compensation is performed by dynamically measuring the current and voltage supplied to the Metselaar control device by its power supply to determine a nominal value of power supplied to the control device and controlled hardware (contactors, etc.). The determined nominal power supply value is then used to select a corresponding temperature offset value from a predetermined table of such temperature offset values. These offset values have been predetermined to correlate the power supplied to the control device with the overall heating that is expected to correspondingly occur within the control device. The selected offset value is then subtracted from the temperature value obtained by the temperature sensor within the control device housing to compensate the measured temperature value for the overall heating within the control device.

While these prior art techniques, and particularly Metselaar techniques, have improved the accuracy of operation of intelligent thermostats, they still produce results with less accuracy than would otherwise be desirable. For example, the actual power consumption of the controlled subsystem/hardware may vary significantly between facilities, affecting the basic assumptions (and hence accuracy) used to create the predetermined offset value.

Furthermore, when operating in different modes and/or different active and inactive (active) outputs, the difference in actual overall heating of the control device may vary significantly, and therefore measuring only the power supplied to the control device may not provide overall heating temperature compensation with the desired accuracy.

In particular, as is well known to those skilled in the art, many devices, such as solid state relays (such as FET-based relays), generate an overall amount of heat that is proportional to the square of the current flowing through them. Thus, while the overall heat value produced by operation of some components of the control device (e.g., the touch screen backlight) will vary linearly and may be predetermined and stored for calculation of the compensation factor, the overall heat within the solid state relays produced by the resistance of its components and the printed circuit board wiring cannot be easily accurately predicted, as the overall heat will vary with respect to the square (i.e., non-linear) of the current flowing through each solid state relay and associated circuit board wiring, which in turn depends on the load connected to the terminal controlled by the respective relay.

As described above, in HVAC systems, the load connected to the terminals of the control device may often vary significantly between different facilities. For example, in one HVAC system, the load (contactor) connected to the W1 terminal may be 3.5 amps, while in another HVAC system, the load connected to the W1 terminal may only be 1.5 amps. It is apparent that the overall heat generated by a solid state relay and circuit wiring handling a 3.5 amp load is more than five times greater than the overall heat generated by the same solid state relay and circuit wiring when handling a 1.5 amp load.

Thus, it has proven difficult to provide overall heating temperature compensation at a desired level of accuracy in prior art control devices, such as intelligent thermostats, and while less than optimal results, various assumptions and/or averages have been employed in the past to make such compensation calculations for such systems.

In contrast, the present inventors have determined that by measuring the actual current provided to the associated terminals of the relay assembly 140, a number of advantages may be achieved, including obtaining a more accurate determination of the overall heat generated in the control device 20, thereby allowing for a more accurate overall heating amount compensation for the temperature measured by the environmental sensor 112.

Turning now to fig. 3, the relay assembly 140 is shown in greater detail. The relay assembly 140 is connected to one or more ports of the processor 100 via the signal bus 122 and may have various output terminals for connection to associated HVAC equipment. In the drawings, only the W1 terminal, the W2 terminal, the Y terminal, and the G terminal are shown for clarity, but it is apparent to those skilled in the art that various other terminals may be included as needed. The relay assembly 140 also includes at least one of an Rh power terminal and an Rc power terminal or equivalent terminal that provide power (i.e., Vcc) to the relay assembly 140 and, thus, to the control device 20.

Each output terminal (W1, W2, Y, G) of the relay assembly 140 is controlled by a

respective switch

144, 148, 152, and 156, which switches 144, 148, 152, and 156 are FET-based solid state relays controlled by the processor 100 in this embodiment. Each

switch

144, 148, 152, and 156 has a respective enable signal (W1On, W2On, Y On, and G On) as part of the signal bus 122.

Vcc is provided to the

switches

144, 148, 152 and 156 via one of two power buses, specifically, an Rh bus 160 and an Rc bus 164. The power supplied to the Rh bus 160 flows through the current monitoring device 168, and similarly, the power supplied to the Rc bus 164 flows through the current monitoring device 172.

Although two power buses each having a corresponding current monitoring device are shown in fig. 3, the invention is not so limited and it is contemplated that in alternative embodiments, the relay assembly 140 may include a single power bus and associated current monitoring device or, if desired, three or more power buses having corresponding associated current monitoring devices may be employed, including implementations having a separate power bus and associated current monitoring device for each output terminal.

It is also contemplated that, in addition to the power buses having associated current monitoring devices, the relay assembly 140 may include one or more power buses that do not include associated current monitoring devices for situations in which the power requirements of the outputs powered by these power buses are fixed or otherwise known and thus the current supplied to these outputs is predetermined or otherwise known.

Returning again to fig. 3, as noted above, Rh bus 160 is connected to the Rh power supply terminal by a current monitoring device 168, which may be any suitable device for monitoring current as will occur to those of skill in the art. In the illustrated embodiment, the current monitoring device 168 provides an output voltage signal rhcount to the processor 100 via the signal bus 122, and the rhcount voltage signal is proportional to the current flowing through the current monitoring device 168 to the Rh power bus 160 and the

switches

144 and 148 connected thereto. The current monitoring device 168 also preferably, but not necessarily, provides a fault logic signal RhFault (discussed below) to the processor 100 via the signal bus 122.

Similarly, the Rc bus 164 is connected to the Rc terminal by a current monitoring device 172 similar to the current monitoring device 168, the current monitoring device 172 providing an output voltage signal RcCurrent to the processor 100 via the signal bus 122, and the rhcount voltage signal being proportional to the current flowing therethrough to the Rc bus 164 and the

switches

152 and 156 connected thereto. The current monitoring device 172 also preferably, but not necessarily, provides a fault logic signal RcFault (discussed below) to the microprocessor 100 via the signal bus 122. In the illustrated embodiment, Rc also provides operating power (Vcc) to the rest of control device 20, and supplies this power prior to current measurement device 172.

For example, in operation, processor 100 may close switch 144 by asserting the "W1 On" signal line, causing the W1 terminal to be connected to the Rh terminal, and thus powered via Rh bus 160. Similarly, asserting the "G On" signal line closes switch 156, connecting the G terminal to the Rc bus 164.

By monitoring the currents measured by the

current monitoring devices

168 and 172, the processor 100 may calculate the amount of current flowing through the

respective power buses

160 and 164 to the

switches

144, 148, 152, 156, etc., and may calculate the amount of overall heat generated by the current flowing through the respective switches and the corresponding PCB wiring.

In a simple case, the processor 100 may have a predefined total resistance value associated with each

switch

144, 148, 152, and 156, each predefined total resistance value being the sum of the corresponding PCB circuit routing resistance and the resistance across the corresponding switch.

In such a case, the processor 100 first determines the total amount of current flowing through each enabled switch (as described further below) reported by the respective current monitoring device. When the respective current flowing through each enabled switch is determined, the processor 100 may calculate the overall heating generated within the relay assembly 140 for each enabled switch by determining the product of the square of the current distributed to the enabled switch multiplied by the predetermined total resistance value of the respective enabled switch and summing these determined values.

Once the overall heating generated within the relay assembly 140 is determined, this value is added to the overall heating generated in the remainder of the control device 20 to obtain a determined sum of the overall heating in the housing. This latter value is determined by determining the voltage and current supplied to the remainder of the control device 20 (these values are determined by other current and voltage sensors not shown), and the product of these determined values represents the overall heating generated in the control device 20, rather than the overall heating generated in the relay assembly 140. The determined sum of the bulk heating values in the housing is then used to select a compensation factor to be applied to the temperature measured and reported by the environmental sensor 112 to obtain a compensated temperature value that is representative of the temperature of the environment surrounding the control device 20 and that is used by the control device 20 when the control device 20 is employed to manage the ambient temperature and/or relative humidity.

Determining the distribution of the measured current through each bus to the corresponding switch enabled on the bus is accomplished as follows. Fig. 4 shows a representation of the ideal current flowing through the bus 160, which in this example includes only two

switches

144 and 148. It can be seen that the total current I that flows_RhBusIs the current (i.e., I) flowing through the switch 144_W1) If any, and the current (i.e., I) through switch 148_W2) If any, is added. Thus, in an ideal case, the overall heating produced by the bus 160 is equal to the square of the current through the switch 144 (I)_W1 ²) Multiplied by the total resistance of the switch 144 and its associated PCB wiring (collectively referred to as R)_W1) Plus the square (I) of the current through switch 148_W2 ²) Multiplied by the total resistance of the switch 148 and its associated PCB wiring (collectively referred to as R)_W2). In this ideal case, if I is measured when only a single switch is enabled_RhBusThen flows through the switchIs equal to I_RhBusAnd may be measured and stored for subsequent use to determine the measured current I between respective switches when more than one switch is enabled_RhBusTo be properly distributed.

Thus, for example, if I is measured when only switch 144 is enabled_RhBusAt 2A, the current I is considered_W1Is 2 ampere (I)_W1＝I_RhBus) And the processor 100 will store the value for future use. When the switch 148 is then also activated and the current monitoring device 168 measures the current I_RhBusAt 3.5 amps, the processor 100 retrieves the current I_W1And subtracting the stored value from the measured value to determine the current I (i.e., 2 amps)_W2At 1.5 amps, and then these values are used in turn to make the bulk heating calculation described above.

However, many, if not most, HVAC systems employ inexpensive transformers that have significant internal resistance (i.e., copper losses, etc.), and the voltage they are supplied to decreases as the power drawn from the transformer increases. Thus, in order to obtain a more accurate determination of the current flowing through each switch, in a preferred embodiment of the invention, this variability in the supply voltage is also taken into account when determining the current flowing through each enabled switch.

For example, the nominal value of the supply voltage on RhBus (VRhBus) is 24 VAC. When switch 144 is enabled and current flows through it to the load, I may be measured_W1Is 2A and VRhBus can be measured as 23.5VAC (measured by a voltage sensor not shown) and these values stored by processor 100. Preferably, this is a dynamic process and is repeated at regular intervals to quickly identify any changes in the operating conditions of the HVAC subsystem and/or the operation of the control 20. In one embodiment of the invention, this process is repeated every 5 seconds.

If switch 148 is also enabled and current flows through it to the load, then IRhBus can now be measured to be 3.5A, and VRhBus can now be measured to be 22 VAC. Based on these values and the previously determined VRhBus and I stored by processor 100_W1The processor 100 may estimate I under these circumstances_W1Is 2A × 22V/23.5V ═ 1.87A, and therefore I_W23.5A-1.87A-1.62A at VRhBus-22V, and processor 100 stores these new values.

It will be apparent to those skilled in the art that if another switch X (not shown) is enabled on RhBus and IRhBus is measured to be 5A and VRhBus is measured to be 20V, then since the processor 100 has stored I when VRhBus is 22V, I is already stored_W1Value of 1.87A and I_W2So processor 100 may determine I when VRhBus is 20V_W1＝1.87A×20V/22V＝1.7A，I_W21.62 Ax 20V/22V ═ 1.47A and I_X5A-1.7A-1.47A-1.83A and processor 100 will also store these values.

Similar calculations are performed when the switch is deactivated. In the above example, when switch 148 is deactivated, I is known from previous determinations_W2Is 1.47A and a VRhBus of 23V can now be measured. Thus, the system continues to measure the remaining current and determine I_W11.7 Ax 23V/20V 1.955A and I_X＝1.83A×23V/20V＝2.1045A。

It will be apparent to those skilled in the art that the determined current value may not be the exact sum of the measured currents reported by current monitoring device 168 and current monitoring device 172 for a variety of reasons (i.e., rounding errors, drift, etc.). Thus, each time current monitoring device 168 and current monitoring device 172 report a measured current to processor 100, the difference between the sum of the determined currents and the measured current on each power bus is proportionally added to the determined current flowing through each respective switch. In other words, if switch 144 and switch 148 are active on RhBus 160, and it has been determined that I is active_W1Is 1A and has determined I_W22A and the current reported by current monitoring device 168 is 3.1A, processor 100 adds 2/3.1 × 0.1A to I_W2Determination of the value to obtain I_W22.0645A, and 1/3.1 × 0.1A was added to I_W1Determination of the value to obtain I_W1＝1.03225A。

This adjustment of the determined current is preferably made at regular intervals (every 5 seconds in the current implementation of the controller 20) and each time the switch is activated or deactivated on the respective bus.

It will now be apparent that by determining the current flowing through the solid state switches of the relay assembly 140 and calculating the overall heating generated thereby within the housing 24, and combining this value with the determined value of the overall heating generated elsewhere within the housing 24, the control device 20 can compensate the temperature measurements taken by the environmental sensor 116 with high accuracy.

Another advantage of the present invention is that improved over-current protection can be provided for the switches in the relay assembly 140. In particular, switches such as the solid state relays 144, 148, 152, and 156 may be damaged by overcurrent events, and even brief overcurrent events may cause fatal damage to solid state relays such as those implemented with FETs. Thus, processor 100 may monitor I_RhBusAnd I_RcBusAnd the corresponding switch may be deactivated in case the respective measured current exceeds a preselected maximum allowed current value. This may reduce the probability that the

switches

144, 148, 152, and 156 will be damaged due to an event such as an electrical short at the time of a faulty installation or other special condition that results in a temporary over-current condition.

One of the challenges in implementing such over-current protection schemes is that the control device 20 is expected to function when connected to a variety of HVAC devices and subsystems. Typically, such HVAC devices are controlled by contactors or other control devices, which are inductive loads on the output terminals of the relay assembly 140. It is well known that inductive loads tend to produce voltage spikes when power is removed due to collapse of their induced magnetic field. In HVAC systems that typically operate at nominal 24VAC, it is not uncommon for such voltage spikes to reach or exceed 100 VAC.

Therefore, the control device 20 needs to be able to reliably distinguish between unexpected fault conditions that may cause damage to the switches and/or connected equipment and normal operating conditions (transient) overcurrent events that may be tolerated. Upon detection of a fault condition, the control device 20 will then deactivate the affected switch quickly enough to prevent damage to the switch and connected equipment.

Ideally, any protection system should operate so as to: deactivating a switch experiencing an overcurrent condition before any damage occurs to the switch or connected equipment; deactivating the switch if the steady state current flowing through the switch exceeds a predetermined threshold; allowing the switch to remain active subject to a transient overload condition that does not damage the switch or connected equipment; and to allow switches that are subject to repeated but not frequent enough overload current transients to remain active.

Since damage to the FET can occur when the FET must dissipate too much power over an extended period of time, raising the operating temperature of the FET junction to a level that causes the junction to be damaged, the amount of power dissipated and the time for which the FET dissipates that power must be considered.

The present inventors have determined that it is not practical to use a single fixed threshold for detecting an over-current event to address all desired protection scenarios. Instead, the inventors have established a set of thresholds that are triggered as described below.

First, the inventors investigated the specific characteristics of the FETs employed in

switches

144, 148, 152 and 156 to determine the maximum power level that the FETs can dissipate in a given time without causing the FETs to exceed an operating temperature that would subject them to damage. Such information is typically available from data tables provided by the FET manufacturer or may be determined empirically, etc.

In the control device 20, the currents through each bus determined by the respective

current monitoring device

168 and 172 are reported to the processor 100 at regular intervals, and in this implementation, these currents are reported every 1 ms. It is assumed that if the current flowing through the respective bus exceeds a predetermined threshold, all enabled switches on that bus should be disabled to ensure that no damage is caused to the switches or attached equipment, as described below. It will be apparent to those skilled in the art that this is a conservative protection strategy in that only one switch may be supplying excessive current and thus susceptible to damage if two or more switches are enabled on a bus determined to be in an overcurrent condition, but the inventors have adopted a conservative strategy of disabling all enabled switches on the bus as this requires less processing and can therefore be achieved more quickly. It is obvious that a strategy of deactivating only the switches that are supplying the excess current may be employed instead if additional computational resources are available or if the expected failure modes are very different.

In a specific example, it was determined that FETs in switches (144, 148, 152, 156, etc.) at an expected nominal operating temperature can tolerate the following peak currents based on a 60Hz AC cycle time: 15.6 amps for an 1/2AC sine wave (cycle); 13.36 amps for a full AC sine wave (cycle); 12.2 amps for two full AC sine waves; 11.2A for four full AC sinusoids; and 9.96 amps for eight full AC sine waves.

From this information, the inventors determined that the maximum measured bus current (I) is acceptable_peak) Five safety threshold value sets of (1):

(1) if I of the bus_peakIf the current exceeds 12.5 amperes, immediately stopping the corresponding bus switch;

(2) if for a complete AC cycle, I of the bus_peakLess than 12.5 amps but greater than 12 amps, then the corresponding bus switch is disabled;

(3) if for more than two complete AC cycles, I of the bus_peakLess than 12 amps but greater than 11 amps, then the corresponding bus switch is deactivated;

(4) if for more than four complete AC cycles, I of the bus_peakLess than 11 amps but greater than 10 amps, then the corresponding bus switch is deactivated;

(5) if for more than seven complete AC cycles, I of the bus_peakLess than 10 amps but greater than 3.5 amps, then the corresponding bus switch is deactivated; and

(6) i of the bus_peakBetween 0 and 3.5 amps is considered the nominal operating state.

In when I_peakIf the lower safety threshold less than the maximum allowed number of cycles is exceeded before increasing to the higher safety threshold, the cycles during which the lower safety threshold is exceeded will be exceededThe count of the number is added to the count of cycles that have elapsed at the higher safety threshold. For example, if I_peakTwo cycles of 5 amps (of seven cycles allowed by threshold (5)), and then I_peakIncreasing to 10.5 amperes, then at I_peakThreshold (4) is considered triggered only if it remains above 10 amps for two additional cycles (a total of four AC cycles defined for threshold (4)).

Furthermore, in any case where the safety threshold is exceeded but not triggered, i.e. I_peakIs 10.5 amperes, I, for one cycle_peakIt must be below the nominal value (i.e., 3.5 amps in this example) for a predetermined number of AC cycles (7 cycles total in the current implementation) to allow the semiconductor junction in the FET to cool to a safer level.

It will be apparent that the actual values used in the safety thresholds will vary based on the particular FETs employed in the

switches

144, 148, 152 and 156, the expected ambient temperature in which the control device 20 is to operate, the AC frequency (60Hz or 50Hz), etc., and that within the normal scope of those skilled in the art of semiconductor device circuit design, the maximum power consumption level for a particular set of FETs and/or a particular set of expected operating conditions can be determined to produce appropriate values for that set of safety thresholds.

Further, it is contemplated that a lesser or greater number of safety thresholds may be defined within the group as desired, depending on the anticipated operating conditions.

In operation, the processor 100 monitors the current flowing through each bus (RhBus 160 and RcBus 168) and compares the maximum current value monitored (and the number of AC cycles it has on the level of interest) to a predefined set of safety thresholds. When the current through a switch exceeds a safe threshold, the processor 100 will deactivate the enabled switch on the corresponding switch to prevent the FET in the switch on the bus from overheating and thus being damaged.

As an additional safeguard, each of the current monitoring devices 172 and 176 preferably has a logic output Rc, respectively_faultAnd Rh_faultThe logic outputs Rc_faultAnd Rh_faultIs active when the current through the respective current monitoring device is high enough to saturate the sensors in the device. In a preferred embodiment, processor 100 couples Rc to a processor_faultAnd/or Rh_faultWhen an interrupt is serviced by the processor 100, and the interrupt service routine may immediately begin to deactivate all switches in the relay assembly 140 connected to the respective Rh bus 160 or Rc bus 164 whose corresponding

current monitoring device

160, 164 reported the fault. After the respective switch has been deactivated, the processor 100 may attempt to re-activate the switch in a safe manner.

Processing R by processing R as an interrupt rather than as an analog input signal that must be digitized by an A/D converter (provided as part of processor 100 or as a separate component)_cfaultAnd R_hfaultThe processor 100 can turn off the relevant switch much faster than if the sample and hold cycle of the a/D converter had to be performed. Thus, in the event of a severe overcurrent event, shutdown can be achieved at a very fast rate.

When it is desired to deactivate the switch due to a fault or other undesirable condition, it is desirable that the closing of the switch occur quickly. It is well known that when the gate voltage to a FET is removed, to turn the FET off, the FET goes from a saturated mode of operation through a linear mode of operation and then turns off. Because of the resistive nature of a FET, it consumes a significant amount of power when in linear mode, it is important that the FET transition through the linear mode region as quickly as possible under overcurrent conditions.

Fig. 5 shows a schematic diagram of the switch 144 of the control device 20. As shown, the switch 144 includes two

FETs

200 and 204 connected between VRhBus and the load. In the controller 20, the voltage used to power the processor 100 and other digital components is 3.3V, while the voltage required to activate the gates of the

FETs

200 and 204 is 10V.

Thus, the controller 20 employs the charge pump circuit 208 to power the gate of each switch. The charge pump circuit 208 receives the PWM voltage 212 from the processor 100 and provides a voltage between 0 and an appropriately selected maximum voltage to the gate of the switch 144 according to the duty cycle of the PWM voltage 212. As will be apparent to those skilled in the artThat is, the PWM voltage 212 corresponds to W for enabling the switch 144 via the signal bus 122_1ONA logic signal where a zero duty cycle corresponds to the switch 144 being disabled and a suitably selected duty cycle (e.g., 80%) corresponds to a suitable voltage to enable the switch 144.

While the charge pump circuit 208 and the corresponding charge pump circuit associated with each other switch in the relay assembly 140 provide the necessary control of the gate voltage of the corresponding FET, they have the disadvantage that the capacitor employed as part of the charge pump circuit 208 and the capacitance of the PCB wiring connecting it with the other components of the charge pump circuit 208 prevents the output of the charge pump circuit 208 from moving to zero voltage quickly enough to move the connected FET through its linear region of operation as quickly as necessary when the switch 144 needs to be deactivated.

Accordingly, the charge pump circuit 208 is also provided with a fast turn-off circuit 216, the fast turn-off circuit 216 operating such that: when the output voltage of the charge pump 208 drops below a threshold, slightly below the normal "start-up" voltage applied to the active gates of the

FETs

200 and 204, the fast turn-off circuit will act to connect the positive output of the charge pump circuit 208 to the negative output, thereby quickly draining any remaining charge in the charge pump circuit 208 and the respective FET gates, and bringing the voltage output to zero and thereby quickly moving the

FETs

200 and 204 through their linear region of operation to their off states.

Each switch in relay assembly 140, including FET-based switches, is similarly equipped with an equivalent charge pump circuit and a fast turn-off circuit, such that when the one or more predetermined thresholds described above are exceeded, processor 100 can turn off the respective switch in relay assembly 140, and the turn-off will occur fast enough to prevent damage to the respective FET.

It should now be apparent that the present invention provides a novel control device, such as an intelligent thermostat, that employs a solid state relay as a switch to activate and deactivate systems, such as HVAC fans, compressors, etc., controlled by the device. Current flowing through the solid state relays to at least some of the controlled systems is monitored to determine an overall amount of heat generated in the solid state relays and their associated circuitry and printed circuit board wiring, and this determined overall amount of heat is used, along with other determined overall heating values, to compensate for readings provided by temperature sensors within the control device affected by the overall heat.

Furthermore, by measuring the current flowing to each power bus in the controller 20, an overcurrent condition that may cause damage can be distinguished from an allowable transient overcurrent condition, and the control device can disable any solid state relays that will be damaged while allowing the solid state relays that are undergoing the allowable transients to remain operational. In the event of a severe overcurrent condition, the current monitoring device may issue a fault signal, triggering an interrupt condition, which will cause the processor in the controller to close the affected solid state relay very quickly.

The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.

Claims

1. A control apparatus for controlling one or more electrical subsystems according to a control program, comprising:

a housing;

a memory storing an operating program;

at least one environmental sensor within the housing for determining a temperature within the housing;

at least two solid state relays, each of the at least two solid state relays operable when activated to connect a respective electrical subsystem to a power source and further operable when deactivated to disconnect the respective electrical subsystem from the power source, each of the at least two solid state relays having a predetermined respective resistance;

at least one current measurement device for determining a total current flowing from the power source to the respective electrical subsystem when the respective solid state relay is activated; and

a processor operable to execute the operating program to: determining a proportion of the determined total current flowing through each of the at least two solid-state relays; determining an overall heating amount generated by the determined total current proportion and the corresponding predetermined resistance, thereby obtaining a determined overall heating amount; adding the determined total heating amount to the total heating amount generated in the housing by other components to obtain a determined sum of the total heating in the housing; compensating the determined temperature value by an amount corresponding to the determined sum of the overall heating in the housing to obtain a compensated determined temperature value.

2. The control device of claim 1, wherein each respective electrical subsystem is part of an HVAC subsystem.

3. The control device of claim 2, wherein each respective solid-state relay comprises: a pair of FETs; a charge pump for supplying a control voltage from the processor to the gate of the FET; and a fast turn-off circuit operable to ground an output of the charge pump when the fast turn-off circuit detects that the control voltage is less than a predefined value.

4. The control device of claim 1, wherein the processor monitors the value of the determined total current reported by the at least one current measurement device and compares that value to a set of predefined thresholds including a current value and an allowed time value, and if the threshold is exceeded for more than the corresponding allowed time value, the processor deactivates the at least two solid state relays.

5. A method of controlling an HVAC system including at least two electrical subsystems using a control device:

measuring a temperature using a temperature sensor located within a housing of the control device;

determining a sum of the overall heating produced by the components and power flow within the housing, comprising the steps of:

measuring current flowing through at least two solid state relays to respective electrical subsystems, each respective electrical subsystem having a predetermined respective resistance;

determining a proportion of total current flowing through each of the at least two solid state relays;

determining an overall heating amount generated by the determined total current proportion and the corresponding predetermined resistance, thereby obtaining a determined overall heating amount;

determining an overall heating value corresponding to overall heating by other components within the housing;

summing all of the determined bulk heating values to obtain a total bulk heating value;

the measured temperature is compensated in view of the determined total overall heating value to obtain a compensated temperature value.

6. The method of claim 5, wherein each respective solid-state relay comprises: a pair of FETs; a charge pump for supplying a control voltage from a processor of the control device to the gate of the FET; and a fast turn-off circuit operable to ground an output of the charge pump when the fast turn-off circuit detects that the control voltage is less than a predefined value.

7. The method of claim 5, wherein a processor of the control device monitors a value of the determined total amount of current reported by at least one current measurement device of the control device and compares the value to a set of predefined thresholds including a current value and an allowed time value, and if the threshold is exceeded for more than the corresponding allowed time value, the processor deactivates the at least two solid state relays.

8. A method of protecting at least one solid state relay connected between a load and a power bus within a control device from potentially damaging overcurrent conditions, comprising the steps of:

(a) determining a set of at least three safety thresholds, each threshold comprising a maximum current level and its maximum allowed time, the thresholds defined in terms of a safe power consumption capability of the solid state relay;

(b) measuring a current flowing through the power bus;

(c) comparing the measured current to each of the at least three safety thresholds;

(d) determining whether the measured current exceeds a maximum current level of at least one of the at least three safety thresholds;

(e) if the measured current exceeds the maximum current level of the at least one of the at least three safety thresholds, determining if the current has existed for a period of time exceeding a maximum allowed time defined for that threshold, and closing each of the at least one solid state relays if the maximum allowed time threshold has been exceeded;

(f) repeating step (e) for each of the at least three safety thresholds; and

(g) repeating steps (b) through (f).